This invention pertains to the determination of sucrose concentration using spectroscopy, polarimetry, and multivariate regression.
Enantiomers are mirror-image isomers that have identical chemical and physical properties, making them difficult to separate and quantify. They can be distinguished optically by means of linearly polarized light. Linearly polarized light is light whose electric vector is confined to a particular plane. When linearly polarized light is passed through a sample containing a chiral molecule, one member of the enantiomeric pair rotates the plane of polarized light in one direction while the other rotates the plane of polarized light in the opposite direction. If the enantiomer rotates the plane of polarized light in a clockwise direction (when viewed by looking back at the source), the enantiomer is referred to as being dextrorotatory. If the enantiomer rotates the plane of polarized light counter-clockwise, it is levorotatory. If the sample contains equal amounts of both enantiomers, it is referred to as being racemic and does not rotate the plane of polarized at all.
A polarimeter is a device used to measure the optical activity of a sample. Polarimeters are well known and consist of a light source, a pair of polarizers, and an eyepiece or detector to measure the intensity of the light.
The sugar industry is by far the largest user of polarimetry today. Since the optical rotation of a sugar solution is proportional to the concentration of the solution, polarimetry is used routinely in the sugar industry to determine the concentration of sugar solutions. Sugar, or sucrose, is conventionally determined by polarimetry with the Clerget method, which consists of observing the difference in optical rotation before and after inversion of the sucrose by hydrolysis to glucose and fructose. One problem with the routine use of polarimetry for determination of the concentration of sugar solutions is coloration of the solution with raw sugar samples. In the conventional method, using the sodium wavelength at 589 nm, clarification is needed with many colored sugar samples like those from dark cane, sorghum, and beet products. To get accurate results, colored solutions need to be decolorized before the optical rotation is measured. Clarification and decolorization are typically accomplished by addition of basic lead acetate. Decolorization with lead acetate creates an expensive chemical disposal problem. Recently, because of concerns over the pollution problems associated with lead acetate, polarimetric measurements in the near infrared region, at about 880 nm, are being used with cane juices after conventional filtration. In this region, the chromagenic centers that cause color do not absorb radiation, so the measurement is not affected by the remaining yellow-brown color. This approach requires a near-infrared (“NIR”) polarimeter. While using NIR radiation gets around the color problem, optical rotation decreases with increasing wavelength, so some accuracy may be lost.
The use of a polarized photometric detector in high-performance liquid chromatographic measurement of enantiomeric purity has been described (Yamamoto et al., 1996). The polarizers used were of sheet polarizing metal, which limits the spectral range to about 400 to 700 nm. The use of near-infrared radiation was not mentioned. Furthermore, only a single wavelength was utilized in any given application, which prevents the use of multivariate regression analysis.